Purification of toluene



Dec. 3, '1946, w. L. R|PPETEAU,JR v 2,412,010

I PURIFICATION 0F`TOLUENE Filed April 24, 1944 LLNE/ STOCK l5 INVENTOR WALTER LESLIE R|PPETEAU,JR.

Patented Dec. 3,

PURIFICATION OF TOLUENE Walter L. Rippeteau, Jr., Baltimore, Md., assigner to Phillips Petroleum Company, a corporation of Delaware Application April 2.4, 1944, serial No. 532,493 sciaims. (Cuzco-674) 1 This invention relates to the treatment of aromatic hydrocarbon stocks. and more particularly it relates to the treatment of aromatic stocks stocks containing same and its ultimate puriflcas having narrow boiling rangesv'for the removal of such hydrocarbons as olens, diolefins, etc., whereby the aromatic hydrocarbon is so purified as to meet commercial standards.

The usual methodfor producing toluene includes very close fractionation of the original toluene containing stock to produce a "toluene concentrate." This concentrate may be refractionated one or more times in an attempt to increase the toluene concentration and at the same time to recover the rejected hydrocarbon material for subsequent use. One widely used method for the lfinal purification of toluene concentrates is by acid treatment with sulfuric acid. Such treatment removes olens and dioleflns as polymeric material which may be ultimately separated as a tarry, sludge-like material. However, it is impractical to remove all of the contaminating oleilns or dioleiins by this method because of the high losses of toluene through sulfonation resulting from repeated acid treatments. It has been found that one to two-treatments with 93 per cent sulfuric acid reduce the olen content to a lowvvalue, but not sufficiently low to meet nitration grade specifications. Further treatments result in high toluene losses through sulfonation. Tests have shown that as many as 5 to 7 acid treatments may be necessary to produ'ce nitration grade toluene from certain stocks. Consequently it is desirable to acid treat as few times as possible. ments are preferred and other less destructive means are used for the removal of the nal traces of the unsaturated compounds. s

One method of effecting the removal of the nal traces of the olens and dioleflns remaining after acid washing is to treat with aluminum chloride. active polymerizing catalyst for oleilns and diolens. It also reacts with these unsaturated compounds to form complexes which areof a sludgy or tarry nature and therefore easy to remove. However, it has been found difilcult to obtain consistently satisfactory results when using aluminum chloride alone as a nal refining agent. I have discovered a method for overcoming this ditnculty wherein a consistently uniformY product can be produced.

Generally, one to two treat- Anhydrous aluminum chloride is anv tion to meetnitration specifications.

Still other objects and advantages of my process will be apparent to those'skilled in the art from a careful study of the following detailed description, in which The figure represents a diagrammatic 'arranger ment of apparatus in which the process of my invention may be practiced.

yI have discovered a method for the treatment A of toluene or ofbtoluene concentrates for' the removal of unsaturated hydrocarbons whereby" nitration gradetoluene can be economically and consistently produced. As mentioned hereinbefore one method for the removal of unsaturated hydrocarbons from toluene concentrates is to.

. treat with anhydrous aluminum chloride, but I An object of my invention is to provide a f have found in daily operations thatthe use of aluminum chloride alone cannot be depended upon to produce consistently toluene of the required purity. I have found that by the use of free or metallic iron along with aluminum chloride that the unsaturated content caribe substantially completely removed, in fact so nearly completely removed, that daily production of specification toluene is easily attained.

The treatment, accordingto my invention, may be carried out in a fractionating tower or other suitable vessel wherein the aluminum chloride treating agent may be retained primarily within' the liquid phase and the free iron exhibiting relatively great surface in the vapor phase section of the vessel. In a packed column the free iron may conveniently serve as all or only a portion of the packing material, as desired. By carrying.

out the treatment in a, fractionating column permits the simultaneous treatment and separation of the toluene from the resultant aluminum chloride sludge as well as from a large portion of any free, unreacted aluminum chloride which may be present.

For the production of a nitration grade toluene from an aromatic oil fraction produced by cracking petroleum hydrocarbons, the following series of steps may be followed. First, the aromatic oil fraction is distilled to recover a toluene and lighter fraction. This fraction may be prepared in a conventional bubble cap type fractionating column, packed column, etc., at atmospheric, superatmospheric or even subatmospheric pressure, as desired.A Steam distillation may alsobe employed. Usually, however, it will be desirable f to koperate at atmospheric or slight superatmospheric pressure. In cases where the aromatic oil stock is a product from thermal cracking of light hydrocarbons such as propane, ethane, etc., wherein the cracking operation is controlled pri-y marily for the production of dioleflns, it may be desirable to use vacuum or steam distillation to prepare this toluene and lighter fraction. Steam or vacuum distillation in such cases may'be deof cyclopentadiene sirable because of the possibility of the presence polymer in substantial The aluminum chloride treated toluene is given a final treatment with caustic soda, such as flake amount.v Distillation at atmospheric or higher appear in the toluene and lighter fraction, and! will subsequently haverto be removed along with other oleilns and dioleflns. fractionating at sufllciently low temperature, such as by steam or vacuum distillation, the amount of oleflns and diolens to be removed subsen l quently from the toluene and lighter fraction can be materially reduced.

The second step consists in fractionally distilling the above described toluene and lighter fraction to remove the larger portion of the materials boiling below toluene. The toluene is retained in Consequently, by

the kettle product which is subjected to another I fractional distillation which is intended to separate still more of the hydrocarbons boiling below toluene. Still another fractionation is frequently required to yield a toluene concentrate of the de` sired narrow boiling range, but this material still may contain as much as 6 to 10% of unsaturated compounds calculated as monoolefins. This fraction may be acid treated and'separated from the unused acid and resultant sludge, by conven-V tional methods. This acid treated material is next caustic washed to remove anyfree acid or acid sulfates retained therein. After the caustic washing step, the once acid treated toluene is again distilled and produces a toluene product having less than 1 C. boiling range. This product will meet the boiling range specifications for nitration grade toluene but still contains too high a percentage of unsaturates to meetthe acid wash color test specication (Barrett).

It is at this point that the aluminum chloride treatment is employed for the removal of the iinal traces of oleflns or dioleiins. The acid treated 1 C. boiling range stock is introduced into the kettle of a fractionating column along with about 0.5% to 1.0% of anhydrous aluminum chloride. The fractionating columnmay be a` packed column wherein all or a portion of the packing consists of metallic iron, or the column may be a bubble cap type wherein one or more trays are covered with a suiiicient amount of iron packing, steel wool, or other iron having a relatively large surface. to eiect the desired results. The toluene fraction is distilled in this column, so prepared,

to obtain good contact between the aluminum chloride, metallic iron surface and the toluene stock, and to separate subsequently the toluene from the resultant aluminum chloride sludge.

caustic, to remove any entrained,.dissolved, or volatilized aluminum chloride carried over with the toluene from the aluminum chloride treatingl step. This final caustic treatment also removes any HC1 which may be produced during the aluminum chloride treating step.

The function of the metallic iron in the vapor phase in the aluminum chloride treating step is not entirely understood. It is known, however, that during this treating stepA or fractionation some aluminum chloride is volatilized with the toluene and reiluxes through the column. Possibly all, or at least some of the reaction between the aluminum chloride and the olens apparently occurs in the vapor space of the column. This belief is substantiated because of the deposition of aluminum chloride sludge on the metallic iron packing' material some trays above thekettle. This sludge is lrelatively nonvolatile, consequently,

its presence on the iron packing cannot be readily construed to mean that the sludge is vaporized column along with the toluene and/ or aluminum chloride. I have also found that if this sludge is not present on the metallic iron packing, little or no' removal of oleflns and dioleiins occurs. It was thought at one time that possibly .any packing material which was readily wetted by the sludge and from which the sludge would not drain readily, or would not be washed off easily by the reux would be satisfactory. Several different packing materials were tried in an attempt tof settle this point. It was found, however, that to a certain extent the effectiveness of the aluminum chloride treating was a function ofthe degree to which the packing was wetted by the aluminum chloride sludge and the ease with which the sludge was washed of! the packing by the reflux. However, the iinal conclusion was that none of the packings, other than free iron, tried in conjunction with the aluminum chloride treating step gave consistently good results. It was thus concluded that free, metallic iron acts as a promotor for the aluminum chloride in effecting the removal of oleilns and dioleilns from an aromatic fraction. However, lit is not to be construed that this invention is necessarily to be g limited by this conclusion, since the actual mech- Reilux Acid Weight Packing time, wash per cent Remarks minutes color AlOla GLASS COLUMN Gland ceramic Raschig rings 15 5+ 1.00 Do 15 6 l. 00 Steel wool in kettle liquid.

30 4+ 0. 25 30 5 0. 50 30 l Steel wool in vapor s ace of kettle. 30 4 0.50 Metal cloth support or packing.

30 2 0. 50 Do. 30 1+ 0. 50 D0. 0 1+ 2. 50 Do. 10 0 2. 00 l0 0 0. 25

IRON COLUMN Broken building brick 0 3 1. 00 0 1 l. 00

These data show that it is possible to get reasonably good olefin and diolen removal using packings other than free iron, however, results obtained were not consistently satisfactory. It may also be noted that the presence of free iron being treated did not give satisfactory results. When the metallic iron was placed in the vapor space of the kettle or used as packing in the co1- umn, the results obtained were consistently satisi'actory. The use of an iron fractionating columnI with broken building brick, the packing other than metallic iron which worked best, gave but little better results than broken building brick supported on a metal screen in a glass column. In other words. it appears that not only is the presence of metallic iron desirable, but for the most effective results it is desirable that it be in a form and so placed as to permit eflcient contact with the toluene vapors.

Referring now to the figure which shows dia# grammatically apparatus in which my invention may be carried out, the toluene containing hydrocarbon stock enters the system by way of a line I, is fractionated in a fractionator 2 in which a toluene and lighter fraction passes overhead. This overhead material then passes through a Iin the liquid phase of the aromatic hydrocarbon line 3 into another fractionator 4 in which the lighter material is removed from the toluene, the latter being withdrawn through line 5 and passed to still another fractionator I4. Toluene is withdrawn as bottoms and passed through a line 5 to a final fractionator l from which the toluene containing from 6 to 10% unsaturated hydrocarbons of substantially the same boiling points as toluene passes oi as an overhead product. This overhead toluene material is conducted by a line 8 to a sulfuric acid treater 9 forremoval of the major portion of the unsaturated materials. From this treater the toluene passes by a line I0 into a caustic washer I I for removal of acidic materials. From the caustic washer the toluene passes by a line I2 to a fractionator I3 from which the toluene passes olf overhead as a product having a boiling range of less than 1 C.

This 1 C. boiling range'toluene containing a small amount of unsaturated constituents passes from the said fractionator |3 byway of a line I5, a portion of the stream being passed by a pump I6 through a line 33 into an aluminum chloride pot Il in which the toluene stock picks up fresh aluminum chloride. The stock and some aluminum chloride then pass by way of a feed surge tank I 9, a line 20 finally into a fractionator 2|. This fractionator may be a packed column type or a bubble cap type as hereinbeforementioned, but in either case the column carries a metallic iron contacting material 22. Aluminum chloride,` aluminum chloride sludge and toluene may be Withdrawn from the bottom of the fractionator 2| and recycled through a line 23 into4 the toluene feed line i5. The original toluene feednot diverted through the aluminum chloride pot I1 passes on in line I5 into the feed surge tank I9. The recycle material from the bottom of the fractionator 2| which enters line I5 by way of said line 23 is also split into two portions, one passing with the feed toluene into the pot while the remalnder passes with the remainder of the feed toluene into the surge tank I 9. Reject aluminum chloride sludge may be withdrawn from fractionator 2| by way of a line 32 for disposal as desired.

The purified toluenepasses overhead from the said contactor-fractionator 2| through a line 24,

vis condensed in a cooler 25 and accumulated in uene is transferred by a pump 21 through a line 29 into a ilake'caustic contactor 3|), a portion, however, being recycled through a line 23 into the top of the fractionator 2|,as reflux. From the flake caustic contactor the purified toluene passes to a storage vessel 3| as nitration grade toluene having an acid wash color value of substantially 0.

Such conventional auxiliary apparatus and equipment as coolers, heaters, condensers, pumps,

valves, 'automaticcontrols etc., all of which arewell known in the art, are, for'purposes of simplicity, not shown inthe drawing. The fractionating equipment, acid treater, caustic washer -and flake caustic contactor are also of standard design and the operation of Iwhich is well known in the art; A tar trap may be used in line I0.

Similarly, such operating terms as refluxA time, which term refers to the difference between the time at which reiluxing begins and the time at which the rst condensate is received, and such terms as acid wash color, which indicate the proportion or amount of unsaturated hydrocarbons in a standard volume of toluene, are well known to those skilled in the art,

The process as herein described should not be limited to the separation and puricationof toluene since the method may be applied with equal success to the production of other aromatics, as for example. benzene, xylenes and others.

What I 'claim is:

l. A process for refining 'a close cut aromatic hydrocarbon fraction containing undesirable um' saturated hydrocarbon impurities, which coml prises distilling the aromatic hydrocarbon frac- .tion in the presence of anhydrous aluminum chloride and metallic iron whereby the unsaturated lhydrocarbon impurities combine withthe aluminum chloride to form a sludge-like material, the metallic iron being disposed in the vapor phase portion of the distillation apparatus and the aluminum chloride disposed in the liquid phase portion, said metallic iron promoting the formation oi? said sludge-like material. Y

2. A process for preparing nitration grade.

toluene from a close cut toluene fraction containing unsaturated hydrocarbon impurities, which comprises distilling the toluene fraction in the presence of anhydrous aluminum chloride and metallic iron whereby the unsaturated hyf drocarbon impurities combine with the aluminum chloride to form a sludge-like material, the me-v tallic iron being disposed In the vapor phase portion o1' the distillation apparatus and the aluminum chloride disposed in the liquid phase portion, said metallic iron promoting the formationy oi' said sludge-like material, removing the distillate and contacting same with a caustic alkali and iinally recovering the nitration grade toluene.

3. A unitary process for preparing nitration'f grade toluene from hydrocarbon charge stocks containing toluene comprising passing the toluene containing stock to a fractional distillation zone whereby a toluene cut is separated having a very narrow boiling range and containing unsaturatedhydrocarbon impurities, treating said close cut fraction with sulfuric acid to remove the maior portion of said impurities, fractionally distilling said acid treated material to produce a toluene stock having a boiling range within about 1 C.. distilling said close cut-toluene stock in the presence of anhydrous aluminum chloride and metallic iron whereby substantially the remaining unsaturated hydrocarbon impurities combine with 7 the aluminum chloride to form a sludge-like material, the metallic iron being disposed in the vapor phase portion of the distillation apparatus andthe aluminum chloride disposed in the liquid phase portion, said metauie iron promoting the formation of said sludge-like material, removing the toluene distillate and' treating same with caus- .tic alkali and finally recovering the nitration grade toluene.

4. A process for preparing nitration grade toluene from a close cut sulfuric acid treated hydrol. carbon traction containing toluene, and some unsaturatedv hydrocarbon impurity, the acid treated drous aluminum chloride, and metallic iron, said iron having a relatively large surface area per unit-of weight, whereby said impurities combine' -with the aluminum chloride to form a sludgelike material, the metallic iron being disposed in the vapor phase portion of the distillation apparatus and the aluminum chloride disposed in the liquid phase portion, said metallic iron promoting the formation of said sludge-like material, removing the distillate and contacting same with a caustic alkali and finally recovering the nitration grade toluene.

6. A process for preparing nitration grade tolu- I ene from a close cut toluene fraction containing unsaturated hydrocarbon impurities which comprises distilling the fraction in the presence of approximately` 0.5 to 1.0 weight percent of anhydrous aluminum chloride, and metallic iron,

disposed in the liquid phase portion, said metallic iron promoting .the formation of said sludge-like material, removing the distillate and contacting same with a caustic alkali and finally the nitration grade toluene.

5. A process for preparing nitration grade toluene from a close cuit toluene fraction containing unsaturated hydrocarbonimpurities which comprises distilling the fraction in the presence of approximately 0.5 to 1.0 weight percent o1' anhyrecovering said iron having a relatively large surface area per unit of Weight, whereby said impurities combine with the aluminum chloride to form a sludgelike material, the metallic iron being disposed in the vapor phase portion of the distillation apparatus and the aluminum chloride disposed in the liquid the formation of said sludge-like material and the sludge-like material collecting on said metallic iron of largesurface per unit of weight, re-

moving the distillate and contacting same with a caustic alkali, and finally recovering the nitration grade toluene.

WALTER n RIPPETEAU, Jn.

phase portion, said metallic iron promoting- 

